Of all the non-human DNA fragments the team gathered, 99 percent of them failed to match anything in existing genetic databases the researchers examined.

With that in mind, Mark Kowarsky, a graduate student in Quake's lab and the paper's first author, set about characterizing all of that mystery DNA.

The "vast majority" of it belonged to a phylum called proteobacteria, which includes, among many other species, pathogens such as E. coli and Salmonella. Previously unidentified viruses in the torque teno family, generally not associated with disease but often found in immunocompromised patients, made up the largest group of viruses.

"We've doubled the number of known viruses in that family through this work," Quake said. Perhaps more important, they've found an entirely new group of torque teno viruses. Among the known torque teno viruses, one group infects humans and another infects animals, but many of the ones the researchers found didn't fit in either group. "We've now found a whole new class of human-infecting ones that are closer to the animal class than to the previously known human ones, so quite divergent on the evolutionary scale," he said.